1. Field of the Invention
This invention relates to devices having a doubly curved crystal for the diffraction of x-rays in spectrometers or instruments for microanalysis and also relates to a method of fabricating such crystal devices with high quality.
2. Prior Art
Doubly curved crystals are known to be useful as a means of focusing monochromatic x-rays or as a wavelength dispersive device in x-ray spectrometers. For example: (1) a toroidally curved crystal can provide point-to-point focusing of monochromatic x-rays, (2) crystals curved to spherical or ellipsoidal shape can be used as dispersive devices for parallel detection of x-rays, and (3) crystals with atomic planes spherically curved and the surface toroidally curved can provide high collection efficiency when used in scanning x-ray monochromators as described in U.S. Pat. No. 4,882,780.
Some of the prior art for doubly curved crystals and their mounting are described in U.S. Pat. Nos. 4,807,268, 4,780,899 and 4,949,367. U.S. Pat. No. 4,807,268 describes a curved crystal for scanning monochromators formed by plastic deformation at elevated temperature and having unique spherically curved planes and toroidally curved surface (this is sometimes called the xe2x80x9cWittry geometryxe2x80x9d after its inventor). The crystals so made have low reflection efficiency and cannot focus to a high degree of accuracy because of the increase of the crystal""s rocking curve width due to the plastic deformation. Subsequent work has shown that in order to preserve a crystal narrow rocking curve width, elastic, not plastic deformation must be used.
U.S. Pat. Nos. 4,780,899 and 4,949,367 describe devices which have crystals elastically bent and bonded to a smooth concave substrate by a thin layer of adhesive. These devices have a serious drawback, namely the smoothness of the crystal surface and crystal planes is strongly affected by irregularities in the bonding layer. The irregularities can result from the lack of uniform initial thickness of the adhesive layer on the substrate or it can occur during mounting of the crystal even if the initial adhesive layer is highly uniform. In addition, the use of a precision concave substrate is disadvantageous because a new substrate which must be made with great precision and expense is required for each new crystal device.
The objectives of the present invention are partly as stated in the previous application, namely: (1) to provide an x-ray crystal device which can be fabricated so that the crystal is doubly curved with a smoother surface and smoother crystal planes than is obtained by other methods of fabrication, (2) to provide an x-ray crystal device whose planes are more accurately curved to a predetermined theoretically-optimum shape, (3) to obtain smaller focal spot sizes when the crystal device is used for focusing x-rays than the spot sizes previously obtained, (4) to provide a method of fabrication that will allow the fabrication of many identical crystal diffracting devices by use of only one mold, and (5) to provide a crystal device that can be aligned for use with a minimum of adjustments, and (6) to provide a crystal device which, when used in x-ray instruments, can be readily removed and replaced with minimal requirement for realignment.
Additional objectives of the present invention are as follows: (7) to provide a simpler means for obtaining the correct orientation of the crystal lamella during fabrication of the x-ray optic, (8) to provide better control of the position of the crystal lamella relative to its mounting plate during fabrication, (9) to provide for an assembly as compact as possible, and (10) to minimize the number of steps required in manufacture.
This invention achieves some of the desired objectives by bonding the crystal to its substrate by a thick bonding agent that has high viscosity in it""s initial state and hardens to a solid in its final state. The crystal is bent to its final state by bending it to conform to a convex mold that has the desired shape of the surface of the crystal using pressure that is applied to the crystal by the viscous bonding agent which receives pressure from a force applied to the backing plate during fabrication. Additional features of the invention include special configurations of the mold containing the surface used for bending, and special characteristics of the crystal and backing plate that make the crystal device more convenient to use and easier to align.